How to iterate name of output image name in blender via python?
Just call RenderSettings.frame_path() which returns the absolute path to the filename for a given frame or concatenate your string like  ".../folder/image_" + str(250) + ".jpg"(error prone due to OS differences).

Proof using the Console:

&gt;&gt;&gt; rd = C.scene.render
&gt;&gt;&gt; rd.frame_path(frame=250)
'/tmp/0250.png'




For multiple render iterations, I'd suggest append the iterator to the filepath:

C = bpy.context
rd = C.scene.render

for r in range(1,3):
    path, ext = rd.frame_path(frame=250).rsplit('.', 1)
    multi_path = path + "_" + str(r) + "." + ext
    print (multi_path)


Terminal Output:

/tmp/0250_1.png
/tmp/0250_2.png